1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries, such as lithium secondary batteries.
2. Description of Related Art
A high energy density battery can be built with a non-aqueous electrolyte secondary battery that uses as a positive electrode active material a layered lithium-transition metal composite oxide, such as a lithium cobalt oxide and a lithium nickel oxide, because such a battery attains a large capacity and a high voltage, about 4 V. A problem, however, with using such positive electrode active materials is that battery capacity degrades if the battery is set aside in a charged state under a high temperature environment.
To solve this problem, such a technique has been proposed that the transition metal site in the lithium-transition metal composite oxide is substituted by a different kind of element. For example, Japanese Patent No. 2855877 proposes a technique for suppressing oxidation decomposition of electrolyte solution on the surface of LiCoO2 and stabilizing crystal structure by adding zirconium to LiCoO2.
However, the addition of zirconium only as described above does not yield sufficient improvements in elevated-temperature storage performance.